SISU Allocation with Indifference among Admission Modalities: a MAX-HRT Model and Comparison of Tie-Breaking Criteria

Candidate allocation systems are central to large-scale selection processes in higher education institutions. In Brazil, SISU distributes vacancies based on ENEM scores and modality rules. This work proposes an alternative model for SISU allocation, assuming that candidates are indifferent between modalities of the same course and prefer only the chosen courses. Under this hypothesis, the problem is formulated as an instance of Hospital/Residents with Ties (HRT), with one-sided ties in the candidate lists, and maximizing allocations corresponds to MAX-HRT restricted to weakly stable solutions. We evaluate four tie-breaking criteria: SISU 2025, Least Competitive First, Observed Deficit First, and RANDOM. Experiments with SISU 2023 data indicate a small impact of the tie-breaking order. RANDOM obtained the largest allocations, and LCF was the best among the deterministic criteria.